In a wireless communication system, a base station communicates with a plurality of remote terminals, such as cellular mobile telephones. Frequency division multiple access (FDMA) and time division multiple access (TDMA) are the traditional multiple access schemes for delivering simultaneous services to a number of terminals. The basic idea underlying FDMA and TDMA systems is based upon sharing the available resource, respectively as several frequencies or as several time intervals, so that several terminals can operate simultaneously without causing interference.
Telephones operating according to the GSM standard belong to the FDMA and TDMA systems in the sense that transmission and reception are performed at different frequencies and also at different time intervals. Moreover, a GSM signal is a phase-modulated signal which has a constant envelope. Stated otherwise, the amplitude of the modulation is constant.
In contrast to these systems using frequency division or time division, the CDMA (Code Division Multiple Access) systems allow multiple users to share a common frequency and a common time channel by using coded modulation. Among the CDMA systems, mention may be made of the CDMA 2000 system, the WCDMA system (Wide Band CDMA) or the IS-95 standard.
In CDMA systems, a scrambling code is associated with each base station making it possible to distinguish one base station from another. Furthermore, an orthogonal code, known as an OVSF code, is allotted to each remote terminal. The remote terminal may be a cellular mobile telephone, for example. All the OVSF codes are mutually orthogonal, thus making it possible to distinguish one remote terminal from another.
Before sending a signal over the transmission channel to a remote terminal, the signal has been scrambled and spread by the base station using the scrambling code of the base station and the OVSF code of the remote terminal. In CDMA systems, it is again possible to distinguish between those which use a distinct frequency for transmission and reception (CDMA-FDD system), and those which use a common frequency for transmission and reception but with distinct time domains (CDMA-TDD system).
In contrast to communication systems having a constant-envelope modulation, like the GSM system, CDMA systems use a modulated signal which has a non-constant envelope. Stated otherwise, such a signal is phase-modulated and amplitude-modulated, and the amplitude of the modulation is not constant.
In a terminal of a wireless communication system, the direct conversion or transposition to a zero intermediate frequency is an alternative to a superheterodyne architecture. This is especially well suited for very highly integrated architectural terminal configurations.
A direct-conversion receiver, i.e., a receiver with a zero intermediate frequency (zero-IF receiver), converts the band of the useful signal directly around the zero frequency (baseband) instead of converting it to an intermediate frequency on the order of a few hundred MHz.
A direct-conversion receiver is the most beneficial radio frequency architecture, not only because it makes it possible to obtain very high levels of integration by reducing the number of external components, but also for its capability of permitting multistandard receptions. Such an architecture unfortunately exhibits some drawbacks, such as a DC voltage offset and second-order distortion.
Two different types of DC voltage offsets have to be considered, namely a static or temporally unvarying offset, and a temporally varying or dynamic offset. The static offset is due to matching errors in the method of manufacturing the receiver, while the dynamic offset stems mainly from the non-linear characteristics and from leakages between the inputs of the mixers of the receiver.
Also, this voltage offset added to the transposed signal may be significant enough to saturate the high-gain amplification stages. This causes a loss of information, thereby appreciably increasing the bit error rate (BER). All the approaches to this problem which have been developed for constant-envelope modulations, such as in GSM systems for example, are not suitable for systems using modulation with a non-constant envelope, such as in CDMA-FDD systems for example.